Which statement about GROUP BY usage in materialized views is true?

• A. All GROUP BY keys must be included in the SELECT list
• B. GROUP BY grouping sets are allowed
• C. GROUP BY ROLLUP is allowed
• D. GROUP BY CUBE is allowed

Answer: (A)

In a materialized view, the grouped columns become the dimensions that define each row of the precomputed result. Those grouping keys need to appear in the SELECT list because the MV stores one row for every distinct combination of those keys, along with the aggregated values. If a grouping key isn’t included in the output, there’s no way to uniquely identify and materialize the groups, which breaks the materialized view’s ability to be refreshed and queried correctly. That’s why every GROUP BY key must be part of the SELECT list when defining a materialized view.

Grouping sets, ROLLUP, and CUBE introduce multiple levels or patterns of grouping in a single query, which isn’t typically supported for materialized view definitions in the same way as a straightforward GROUP BY. Hence they aren’t the correct fit in this context.